Which mechanism in semiconductors is responsible for electron excitation from valence to conduction band in thermal equilibrium at temperature T? Do valence electrons take their excitation energy from the lattice by absorbing phonon? If so, they can be excited at each arbitrary temperature no matter how much the temperature is down! Another relevant problem: I don't know why generation and recombination rates must be the same in equilibrium? electrons can be excited without affecting the thermal equilibrium this way:they receive heat from lattice but since they are not localized, they can be participant in thermal equilibrium (in other words the equilibrium is so existing) and we don't necessarily need to have any recombination processes for the equilibrium condition. In brief, having the recombination process is not the necessary condition for thermal equilibrium,and I think the equilibrium can exist by having only the generation process. could anyone help me please?